In recent years there has been a move towards broadcasting television signals in digital form. Suitable receivers/recorders (also known as ‘set top boxes’, but generally referred to herein as ‘receivers’) have been developed to take advantage of the digital format to allow the viewer, among other things, to record one television programme while watching another. An example of such a receiver is described in our International patent application published as WO-A-01/11865, the teachings of which are incorporated in this document by reference.
The receiver described in that International patent application is arranged to receive signals representing television programmes and television programme schedule data and has a recorder comprising a “hard disk” for recording received television programmes. The receiver constantly receives updated programme schedule data in a dedicated programme schedule data channel and is arranged to output the programme schedule data for display on a television screen. A user can use the displayed programme schedule data to select programmes for recordal.
The receiver is also arranged to receive additional programme schedule information included in each received television channel, which information is used to control the hard disk to record user-selected pre-programmed television programmes. The hard disk is operable to record simultaneously two different television programmes received in different channels. Also, the receiver is operable to replay a currently broadcast programme offset in time. The time offset can be overcome by playing back the part of the programme inside the time offset at an increased frame rate. Furthermore, the receiver is arranged to receive transition signals indicating transitions between parts of programmes. Transitions might be between different items in a magazine format programme, for example a sports magazine programme or music video programme. The recorder is operable to use these transition signals, for example, to skip between parts of recorded programmes replayed from the hard disk.
Some receivers include a facility for recording a programme while it is being viewed to enable “instant” playback of a portion of the programme while it is still being broadcast. Instant playback recording might begin automatically when the viewer first switches to a channel and might continue until such time as the viewer switches from the channel or until an allotted memory space is full up. Recording can be automatic as it can be done without any intervention by the viewer.
Instant playback recording may be implemented by recording a programme as data in a so-called cyclic file, such as described in EP1185095. A cyclic file is generally a data file of fixed size, e.g. a certain number of bytes such as 2 gigabytes. Data may be recorded in a cyclic file as it is received until the file is full of data. At that point the recording continues from the beginning of the file, writing over the data previously recorded in the file. Thus, the cyclic file always contains the most recent viewed portion of a programme as far as the size of the cyclic file allows.
The size of a cyclic file used by a receiver is typically selected such that it can store a length of program likely to be useful to a viewer whilst bearing cost considerations in mind. As memory costs have fallen and sizes of, for example, hard disk memories have increased, the amount of time that can be recorded in cyclic files has increased. At present, a cyclic file may typically be expected to store received video data of approximately 30 minutes to one hour in length, thus giving the viewer the option of instant playback of any portion of a programme from up to one hour previously.
Compression techniques are used to compress digital television data before it is transmitted. A frame of video showing, say, a crowd scene is generally much more complex than a frame of video showing, say, a commentator in front of a plain background and thus will be more difficult to compress without introducing unwanted artefacts. Similarly, video including significant changes between frames, such as video of fast moving scenes, is generally more complex than slower changing video. Less compression can therefore be applied to some video, such as the crowd scene, than to other video, such as the commentator, in order to maintain the same level of perceived quality. To try to maintain the same level of perceived quality, but at the same time compress the video data as much as possible, the rate at which video data is transmitted can therefore be varied depending on content. This is known as Variable Bit Rate (VBR) encoding. For example, during transmission of the crowd scene a data rate of 6 megabits per second may be used, while during transmission of the commentator, only 3 megabits per second may be used. It is therefore not possible to know in advance how much disk space will be required to store data for 30 minutes (say) of a programme when VBR encoding is used.
Furthermore, it is common for broadcasters to vary the bandwidth allocated to a channel in order to accommodate demands on their networks. For example, a particular uplink to a satellite of a satellite network may have a fixed bandwidth, i.e. data rate, of (say) 20 megabits per second available to transmit data to a given satellite. This bandwidth may be divided between the channels broadcast via the satellite, generally such that the full available bandwidth is exploited, i.e. such that there is little or no spare bandwidth. However, the bandwidth required by each channel may vary from time to time. Furthermore, the number of channels to be transmitted on the uplink may change from time to time, e.g. during the course of a day. The bandwidth allocated to a channel and hence the data rate at which particular programmes are received at a receiver may therefore vary. Again, it is therefore very difficult to determine, in advance, the size of a cyclic file that will be required to store 30 minutes (say) of a given programme.
To ensure that a user always has at least 30 minutes (say) of content on the disk, cyclic files have therefore been made as large as may be necessary to cover the worst case, e.g. to make the file large enough to store 30 minutes (say) of video data transmitted with the lowest compression, i.e. at the highest data rate. This is wasteful of disk space. Having to dedicate memory for the greatest possible memory requirement for the cyclic file also limits the user's choice. It may also be unpredictable for a user, as the user is not certain whether the cyclic file contains 30 minutes (say) of a programme, more or less.